Method for making silicon for solar cells and other applications

ABSTRACT

A method for preparation of high purity silicon suitable for photovoltaic cells using reduction of silica, which is pre-purified in an aqueous solution, in presence of a reducing agent, preferably carbonaceous agent, where the pre-purified silica has a low amount of boron suitable for photovoltaic cells is described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/685,899 filed on Mar. 14, 2007 which claims priority under 35 U.S.C.§119, based on U.S. Provisional Application Ser. No. 60/782,361, filedMar. 15, 2006. The teachings of these referenced applications areincorporated herein by reference in their entireties.

BACKGROUND OF THE INVENTION

Rising energy costs and stretched power grids as well as a desire forenergy independence has sparked a recent surge in the use of solarpanels (photovoltaic) to make electricity. Currently, over 90% of solarcells in the market use silicon. However, the lack of an intermediategrade of silicon has hampered the growth of the silicon solar industry.Until recent years, the total demand of silicon for solar cells wassmall enough to be sufficient sustained by left over scrap silicon fromthe electronics and semiconductor industry. The new demand, however, hascompletely outstripped such source of silicon.

Currently, there are two grades of silicon. There is a metallurgicalgrade (MG) used by the steel and metals industry as an alloyingmaterial. This material is made from relatively crude materials (sandand coal or coke) and yields a cheap source of silicon at about 98-99%purity. This is not pure enough for solar grade (SoG) silicon thatrequires about 99.999% (5,9's) or 99.9999% (6,93 s). Some companies(such as Elkem) produce higher purity MG silicon by using aluminuminstead of carbon as the reducing agent. This material is often used tomake electronic or semiconductor grade silicon which is better than99.999999% (8,9's) pure.

The method to make 8,9's silicon is called the Siemens process, whichuses MG silicon as a starting material. The process is very capitalintensive and expensive to run and causes Siemen's silicon to be veryexpensive. Solar cells require very large area of silicon to absorbsunlight so that the cost associated with 8,9′ silicon in solar cells isprohibitive. Silicon produced as waste material during the preparationof the 8,9's silicon often meets the SoG silicon specifications.However, the electronic industry only produces about 4,000 tons per yearof such scrap silicon, which cannot meet the current demand for solarsilicon, e.g., over 10,000 tons per year.

Much effort has been expended to try to upgrade MG silicon to SoGsilicon. The Siemens process does this chemically by reacting MG siliconwith HCl at high temperature. This produces a family of chloro-silanesand other impurities that are then rigorously distilled and purifieduntil only a very pure stream of trichlorosilane remains. This materialwith hydrogen added is decomposed over high purity silicon heatedsilicon to decompose mixture to pure (8,9's) silicon and HCl.

However, solar cells can be made with silicon of a lesser purity. If aspecific process aimed at the 6,9's purity level were developed for SoGsilicon then the solar industry could resume its growth whilemaintaining a competitive edge for electrical generation costs.

Much effort has been put into starting with MG silicon and upgrading it.The Siemens process does this chemically. Many attempts have been madeto use pyrometallurgical processes. However, dealing with molten siliconis difficult and the number of selective tools for purification is few.These tools are primarily, gas reactions, fluxing with solid or moltenmaterials and various methods of direction solidification. All of thesemethods have their limitations and to date no combination of thesemethods has produced a viable commercial method that is used by anymanufacturer of silicon. The one partial success is the HEM (HeatExchanger Method) which is a directional solidification method used toincrease the purity of bulk silicon. However, this method is not usefulfor upgrading MG silicon to SoG silicon. At this time the largestfurnaces available produce about 200 kilograms of useable silicon every50 to 60 hours. The method is slow and consumes much energy. Further,the technique depends on materials (being removed) having a partitioncoefficient significantly less than 1 (typically below 0.1 to beeffective). While many materials do have low partition coefficients forthe solubility difference in molten verses solid silicon, this methodremoves many impurities. However, two materials, boron and phosphorus,are particularly deleterious to solar cells and also have high partitioncoefficient (0.8 for boron) and 0.35 for phosphorus. Thus, the HEMmethod (a directional solidification method) is not a suitable way topurify silicon if these contaminants are present in an amount above thefinal desired tolerable limits. Almost all MG grade silicon has boroncontents (typically >100 ppm) well above the requirements of a few partper million or less. For a high quality photovoltaic cells, siliconhaving a boron content of about 1 ppm or less is often required.

In summary, there is no economical source of solar grade silicon.Metallurgical silicon is too impure and semiconductor silicon is tooexpensive.

SUMMARY OF THE INVENTION

The present invention relates to a method for preparing high puritysilicon suitable for photovoltaic cells using reduction of silica, whichis pre-purified in an aqueous solution, in presence of a reducing agent,preferably carbonaceous agent, where the pre-purified silica has a lowamount of boron suitable for photovoltaic cells, preferably about 5 ppmor less, more preferably about 3 ppm or less, even more preferably about1 ppm or less or less than 1 ppm, even further preferably less than 0.5ppm. The pre-purified silica is preferably obtained by contacting theaqueous silica solution of a water soluble form of silica, preferablyalkali silicate, more preferably sodium silicate or potassium silicate,with a boron removing agent, preferably a boron specific chelatingresin, even more preferably, an ion exchange resin having a functionalgroup of N-methylglucamine. The aqueous solution may also be treatedwith phosphorus removing agent, preferably a transition metal, calcium,or magnesium, or with molybdate salt or molybdate salt treated anionresin. The carbonaceous agent is activated carbon or carbon blackpreferably substantially free of boron and phosphorus, more preferablyhaving a boron content of about 1 ppm or less.

In another embodiment, the present invention relates to a method forpreparing high purity silicon including obtaining an aqueous silicasolution; filtering the solution; contacting the filtrate with an boronspecific ion resin column; converting the silicate to silica; reducingthe silica with carbon having low content of boron and phosphorous in afurnace; and cooling the resulted molten high purity silicon.

In another embodiment, the present invention relates to a methodpreparing high purity silica having low B and P contents where themethod includes obtaining aqueous silicate solution; optionally adding atransit metal, calcium, or magnesium; filtering the solution; contactingthe filtrate with an boron specific ion resin column; removing thewater; and converting the resulted silicate salt to silica.

In another embodiment, the present invention relates to purifying lowgrade silicon to high purity silicon suitable for photovoltaic cells byconverting the low grade silicon to a water soluble form; purifying thewater soluble form of silica in an aqueous solution; converting thepurified water soluble form of silica to silica; and converting theresulting silica to the high purity silicon

DESCRIPTION OF DRAWINGS

FIG. 1 is a part of an illustrative process flow diagram showing anexemplary silicon production process employing the present invention.

FIG. 2 is the second part of the illustrative process flow diagram ofFIG. 1.

FIG. 3 is the third part of the illustrative process flow diagram ofFIG. 1.

DESCRIPTION OF THE INVENTION

What has been overlooked to date is the fact that chemical purificationsare much more easily and economically accomplished in the aqueous phase.No method to date has proposed pre-purification of the ingredients usedin making the silicon. While there have been proposals to use higherpurity compounds (such as quartzite instead of sand), there is notreatment of these materials and it turns out that the natural sources,while potentially purer are not pure enough over the long term as therecan often be inclusions of many minerals, that are sufficient to exceedthe B and P purity requirements.

MG Silicon is made (and has been made for over 100 years) by reaction 1below: (at about 1700 C)

SiO₂+2C→Si+2CO  (1)

Reaction 1 is rather simplified and there are several intermediates andother side reactions as listed below that can occur.

SiO₂+2C→Si+2CO  (2)

SiO₂+3C→SiC+2CO  (3)

SiO₂+C→SiO+CO  (4)

SiO+C→Si+CO  (5)

SiO+SiC→Si+CO  (6)

The above reactions are typical of carbothermal reduction. The productof reactions 1-6 is usually about 98-99% purity.

Silicon can also be made by replacing the carbon with a sufficientlyactive metal (or alloy) such as magnesium or aluminum.

The reaction then becomes

3SiO₂+4Al→3Si+2Al₂O₃  (7)

This reaction is used to make a slightly higher grade of MG silicon(about 99.9% and is used as a source of silicon for the Siemens process)but aluminum is somewhat more expensive than carbon. Further, the sourceof silica is still a key source for impurities such as boron. Once, theimpurities are put in the furnace they will be incorporated in thesilicon, as boron is not volatile, even at these high temperatures.Reaction 7 is referred to as an aluminothermic reduction. It is to beunderstood that there is no theoretical reason (although there clearlyare practical reasons which are addressed by this process) why thecarbothermal (or metalothermal) reaction should not make a high puritymaterial. If the impurity is not put in the furnace in the first placeit will not appear in the finished product. If pure materials arecharged into a clean furnace that is dedicated to only making highpurity silicon then the only other source of impurities would come fromthe furnace liner or carbon rods. By choosing the correct material forthe furnace liner this source of impurity is also eliminated. Thus,graphite, or silica that are already in the raw materials or othercompounds that do not have an interact with molten silicon or thestarting materials such as alumina, alumina chromite, silicon nitride,orsilicon carbide, are typical of available compounds that can be usedfor crucibles or brick type liners. Additional materials such as thoriumoxide, zirconium oxide, zirconium nitride or various zirconates are alsosuitable. Any suitable furnace may be used. However, the design mustallow for the exclusion of air and allow an inert atmosphere.

Typical furnaces used by industry are electric arc, induction orcalciners. MG silicon is made using submerged electric arc furnaces.Since electric arc uses graphite (carbon) electrodes that do come intocontact with the silicon, the purity of the electrode needs to be highso that impurities cannot come from the electrodes. Such electrodes areavailable from UCAR and Graftek. In an induction furnace, heat istransferred in via an A.C. electric field either directly to thematerial or through a conductive susceptor such as graphite. Again, thematerial holding the molten silicon must be clean and made of suitablematerials such as listed above.

In a calciner a rotating tube is heated indirectly by either electricelements and or a flame source. Again, the material that the tube ismade from must not impart impurities to the silicon.

Any suitable furnace will be acceptable for use in this invention asoutlined above. Preferably, the furnace is the bottom-tapped submergedarc furnace.

The key to the high purity is to create high purity silica from anaqueous system. For example, a water-soluble silicate such as sodiumsilicate is obtained. This can be purchased or made by the reaction ofsoda ash and silica at high temperature by the following reaction:

Na₂CO₃+SiO₂→Na₂SiO₃+CO₂  (8)

Sodium silicate may also be made directly in an aqueous solution byreaction of sodium hydroxide and silica as below:

2NaOH+SiO₂→Na₂SiO₃+H₂O  (9)

Of course, other suitable carbonates or hydroxides may also be used suchas potassium hydroxide, potassium carbonate or any other cation thatproduces a soluble silicate.

The silicate solution is then cleaned to get any undesirable impuritiesout. This can be accomplished by a variety of different operations.

Typically the solution is filtered to remove any insoluble matter. Thesolution is tested for various elemental impurities. Of special interestis of course boron and phosphorus. Any suitable testing method isacceptable. The silicate solution will be at a rather high pH. Silicatesare also poor complexing agents. Thus most metals will have very lowsolubility in the silicate solution and will be removed during theinitial filtration step. These metals will include most transitionelements such as iron, nickel, zinc, and cobalt as well as calcium andmagnesium. However, if desired, a polishing step with an alkali metalion such as sodium or proton or ammonium charged cation exchanger canemployed to remove some final elements especially multivalent cationsthat are attracted more strongly to many cation exchange resins thansodium is.

Boron in the solution will be in the form of borates. Such boron can beremoved by using a boron chelating agent, preferably an ion exchangeresin having a functional group of N-methylglucamine. For example,Amberlite®IRA-743, sold by Rohm & Haas having Corporate Headquarters at100 Independence Mall West, Philadelphia, Pa., is a borate specificchelating resin. It functions well at basic pH as well. It has a highaffinity for borate without affecting the silicate. The resin can beeasily regenerated in a two step method as required by using sulfuricacid and aqua ammonia. The borate is removed as boric acid. The resin isrinsed with DI water (de-ionized water) and can be reused. Multiplecolumns may be used in series to get any desired purity level of boron.Standard techniques for ion exchange apply to all IX methods mentionedin this disclosure.

The pre-purified silica should have a low boron content suitable forphotovoltaic cells. Although one of skill in the art would be able todetermine what is the low boron content suitable for photovoltaic cellsin view of the industrial standards in the field of photovoltaic cellsat the time of this application. However, a preferable low boron contentis about 5 ppm or less, more preferably about 3 ppm or less, even morepreferably 1 ppm or less, or less than 1 ppm, further preferably about0.5 ppm or less.

If traces of transition elements or magnesium or calcium are present,much of the phosphorus will be precipitated and be removed by thefiltration step. It is also possible to intentionally add certainmaterial to encourage this precipitation but small amounts of silicatemay also be lost. If the amount of phosphorus in the solution is high,it can be eliminated by using an anion resin that is treated withammonium molybdate or the tungstate and then rinsed. Such a column willspecifically absorb phosphate in a basic solution. The resin dispensedwith molybdate salts or molybdate salts themselves can be used. However,when the unattached salts are used, the molybdate could pass into thesolution although they can be easily removed by an anion resin. Usingthe treated anion resin can reduce a step. Any other conventionalmethods to remove B or P or any elements may also be applied.

All of the above should be preformed in containers made from materialsthat do not leach boron like, for example, borosilicate glass does. Somost conventional plastics and many metals such as stainless steel aresuitable receptacles for the silicate solution.

The sodium silicate solution after being treated as above is now readyto be used. Heating of this solution and/or the addition of acid orother chemicals will start the formation and precipitation of puresilica usually as a gel. The silica is precipitated washed and dried.The silica maybe washed and dried using conventional equipment such ascentrifuges, filter presses and the like. The silica may additionally beprocessed through a furnace to create the desired crystal forms ofsilica such as cristobalite, tridymite, quartz, lechatelierite oramorphous which may be desirable for the reduction step.

Next is to consider the carbonaceous agent for use in this process. Thecarbonaceous agent is preferably substantially free of boron andphosphorus and includes activated carbon or carbon black. If thecarbonaceous agent causes silicon made from the pre-purified silica tohave boron and/or phosphorus contents beyond the acceptable amounts forphotovoltaic cells, the carbonaceous agent would not be substantiallyfree of boron and phosphorus. Also various forms of carbon are known tobe quite pure. Such commercially available forms are carbon blacks thatcan be made from natural gas, ethylene or acetylene. Carbon blacks madefrom residual oils will contain undesirable impurities. If a lower gradeof carbon is to be used it can be suspended in water leached ofimpurities and washed and rinsed and dried. Amorphous carbon, graphiteor various charcoals may also be used. Again the purity is a key item.Charcoal may be made for instance by the pyrolysis of carbon containingmaterials such as sugar. If the material is water soluble like sugar, itmay be dissolved and purified, as the silicate solution is purified, toreduce the impurities to an acceptable level. The purified sugarsolution is now dried and the sugar pyrolyzed to charcoal. The carbonmay be further treated with chlorine. The carbon can also be made from aflammable gas such as natural gas, methane, ethane, acetylene, ethylene,propane, propene, allene, butane, LPG or in general any C1-C4 gases thatare substantially free of boron and phosphorus

Another factor in the efficiency of the reduction reaction in thefurnace is the degree of graphitization of the carbon. This is a measureof the percent of the carbon that is in the graphite form. This may becontrolled by either mixing graphite with an amorphous form of carbon orany form of carbon may be treated in a graphitization furnace to formgraphite. The degree of graphitization is preferably between about30-55%, more preferably about 40-45% but can vary depending on furnaceconditions and feedstocks. Most carbons start to graphitizate at around1200° C. in inert atmospheres or vacuum. The temperature and residencetime may be varied to achieve any degree of graphitization required.Various carbon forms can be mixed to obtain the proper percentage ofgraphite, for example, by mixing 45% graphite with 55% carbon black.

The prepared silica and the prepared carbon may now be further modifiedto be suitable for the size of the arc furnace. Due to the release oflarge quantities of gas (mostly CO) the charge in the furnace must allowthe gases to escape without building up any significant pressure. Thisis achieved by using briquetters to create briquettes of silica andbriquettes of carbon. Several different sizes of briquettes may becreated as desired to achieve smooth and efficient furnace operation.

As an alternative method, silica and carbon may be co-precipitated bythe following procedure. The pure carbon can be added to the puresolution of sodium silicate and suspended carbon that is being stirred.In the presence of the carbon particles, the carbon will provide anucleation site for the silica. This will result in the co-precipitationof a silica-carbon mixture that is very evenly and intimately mixed. Ifsuch a mixture is desired for its intimately mixed properties this or asimilar methodology would be applicable.

The precipitated material is filtered (use of a centrifuge is alsopossible) and rinsed with DI water to eliminate any impurities that aresoluble in water. The filtered material is handled via closed conveyers(so as not to let in any impurities) into a drying oven to dry thematerial to at least 500° C. This ensures that there is no free wateravailable to make steam in the furnace, which could be a safety problem.Further, steam in the furnace also increases the rate of graphiteelectrode consumption. The powder is now conveyed directly to thebriquettes as before and then added to the furnace and heated up to thereaction temperature. The reaction rate will vary with the energy inputof the furnace, as the reaction is very endothermic. The sensible heatis only about 10% of the energy input the rest is for driving thereaction. This input is theoretically about 448 kJ/mole (˜4.5 kWh/kG) ofsilicon. Most arc furnaces consume about 12 kWh/kG.

Since the silica and carbon each contains less than a ppm of anyimpurity the only other impurities possible are carbon and oxygen. Theoxygen can be reduced by using a greater than stoichiometric amount ofcarbon. The carbon's solubility in molten silicon is about 20 ppm. Ingeneral carbon is inert for most photovoltaic applications andordinarily would be acceptable. However, if the silicon is allowed tocool directionally as in an HEM or other DS furnace (usually from thebottom to the top) carbon has a good partition coefficient of 0.05 whichthen results in about 1 ppm of C in the finished silicon. Further,traces of suspended solids such as SiC will also be removed by the DSstep. It should be noted that as an energy saving step, if higher purityis desired the melting and DS (directional solidification) should becombined so that energy is saved since the silicon will not have to bemelted a second time.

As the silicon is tapped from the bottom of the furnace it should betapped in an inert atmosphere. Several gases such as argon, heliumcarbon dioxide and nitrogen are suitable gases depending on the purityrequirements. If tapped in air the oxygen would start to intrude intothe silicon. Nitrogen too can react with silicon, but is slower thanoxygen. Thus, depending on the tapping and pouring conditions nitrogenmay or may not be a suitable gas. However, argon and helium have nochemical reaction at all with silicon and may always be used.

Finally, the molten silicon may be further treated with otherpurification methods such as steam injection to remove carbon as CO. Theadditional treatments in the molten state may also include the additionof rare-earth metals such as misch-metal (primarily cerium) that isactive enough to form cerium carbide, nitride and oxide from carbon,nitrogen or oxygen that may be in the silicon. This carbide will then beremoved during the DS step to follow for wafer manufacture. All the rareearth elements have very low partition coefficients and can beefficiently removed in the subsequent DS step.

Other advantages of the proposed invention will be obvious to thoseskilled in the art. For instance, it is easy to do large-scale treatmentof the silicate solution and economies of scale can be obtained. Itshould also be noted that the high purity silica obtained would also bean excellent feedstock for the aluminothermic process to make silicon.Again, as before fine aluminum particles could be suspended in thesilicate solution and then the silica precipitated around the aluminum.Or the high purity silica can be precipitated separately and then mixedwith the reducing metal such as aluminum or magnesium. However, themetals would have to be checked for impurities.

Other adaptations are as follows. Pure silica is often available in whatis called fumed silica from several mineral processing industries suchfumed silica can also be a suitable feedstock for this process.

The source of carbon can also be varied. For instance ordinary tablesugar can be used. The sugar is heated to decompose all of the sugar toa pure carbon. If the sugar requires removal of boron or otherimpurities a water solution of it can be made and purified in the exactsame way as the silicate solution is purified. The water is evaporatedand the sugar is pyrolyzed Any material that can be pyrolyzed to a pureform of carbon may also be used in this invention. Such other materialsinclude many foodstuffs such as starches, celluloses, oils glycerin andrice hulls.

The following examples illustrate certain aspects of the presentinvention, and are not intended to limit the scope of the invention asdefined by the appended claims.

Example 1

10.0 grams of sodium silicate was dissolved in 200 milliliters ofde-ionized water. The pH of the solution was 11.2 The sample wasanalyzed for boron using both Inductively Coupled Plasma (ICP) as wellas HACH wet boron determination methods using BoroTrace™ 3 reagent. Thestarting boron content was 4 parts per million by weight. The originalsolution was passed through a 50 milliliter resin column containing Rohm& Haas, Amberlite® IRA-743-A chelating resin. The resin uses moiety thatspecially attracts boron. Other companies make a similar suitable resinwhich may also be used in this application. The column does not absorbsilicate or sodium ions. A 75 milliliter fraction was collected andanalyzed for boron. The boron was reduced to 0.1 ppm.

Example 2

10.0 grams of sodium silicate was dissolved in 200 milliliters ofde-ionized water. The pH of the solution was 11.2 The sample pH wasadjusted to 10.0 with sulfuric acid. The sample was analyzed for boronusing both Inductively Coupled Plasma (ICP) as well as HACH wet borondetermination methods using BoroTrace™ 3 reagent. The starting boroncontent was 4 parts per million by weight. The original solution waspassed through a 50 milliliter resin column containing Rohm & Haas,Amberlite® IRA-743-A chelating resin. The resin uses moiety thatspecially attracts boron. Other companies make a similar suitable resinwhich may also be used in this application. The column does not absorbsilicate or sodium ions. A 75 milliliter fraction was collected andanalyzed for boron. The boron was reduced to 0.09 ppm.

Example 3

10.0 grams of sodium silicate was dissolved in 200 milliliters ofde-ionized water. The pH of the solution was 11.2 The sample pH wasadjusted to 10.0 with sulfuric acid. The sample was analyzed for boronusing both Inductively Coupled Plasma (ICP) as well as HACH wet borondetermination methods using BoroTrace™ 3 reagent. The sample wasintentionally spiked with 12.0 milligrams of boric acid. The startingboron content was 15 parts per million by weight. The original solutionwas passed through a 50 milliliter resin column containing Rohm & Haas,Amberlite® IRA-743-A chelating resin. The resin uses moiety thatspecially attracts boron. Other companies make a similar suitable resinsthat may also be used in this application. The column does not absorbsilicate or sodium ions. A 75 milliliter fraction was collected andanalyzed for boron. The boron was reduced to 0.1 ppm.

Example 4

10.0 grams of sodium silicate was dissolved in 200 milliliters ofde-ionized water. The pH of the solution was 11.2 The sample pH wasadjusted to 10.5 with sulfuric acid. The sample was analyzed for boronusing both Inductively Coupled Plasma (ICP) as well as HACH wet borondetermination methods using BoroTrace™ 3 reagent. The starting boroncontent was 4 parts per million by weight. The original solution waspassed through a 50 milliliter resin column containing Rohm & Haas,Amberlite® IRA-743-A chelating resin. The resin uses moiety thatspecially attracts boron. Other companies make a similar suitable resinwhich may also be used in this application. The column does not absorbsilicate or sodium ions. A 75 milliliter fraction was collected andanalyzed for boron. The boron was reduced to less than 0.04 ppm thedetection limit of the ICP method.

Example 5

The sample from Example 1 was acidified with sulfuric acid to pH 7.0.The solution was allowed to stand for ten minutes during which time asilica gel formed. The gel was washed with DI water, filtered and dried.The gel was analyzed for boron and it contained 0.15 ppm B.

Example 6

The sample from Example 4 was acidified with sulfuric acid to pH 7.0.The solution was allowed to stand for ten minutes during which time asilica gel formed. The gel was washed with DI water, filtered and dried.The gel was analyzed for boron and it contained 0.05 ppm B.

Example 7

1000 kilograms of sodium silicate is dissolved in 20000 liters ofde-ionized water. The pH of the solution is 11.2. The sample pH isadjusted to 10.5 with sulfuric acid. The sample is analyzed for boronusing both Inductively Coupled Plasma (ICP) as well as HACH wet borondetermination methods using BoroTrace™ 3 reagent. The original solutionis passed through a 500 liter resin column containing Rohm & Haas,Amberlite® IRA-743-A chelating resin. The resin uses moiety thatspecially attracts boron. Other companies make a similar suitable resinwhich may also be used in this application. The column does not absorbsilicate or sodium ions. A entire fraction is collected and analyzed forboron. The solution from the column is now treated with sulfuric aciduntil the pH is 7.0. After standing a gel forms which is filtered anddried at up to 1400 C. The dried material is briquetted into briquettesof several sizes from about 2-6 inched on edge. Separately, 1000kilograms of sugar is dissolved in 2000 liters of DI water the solutionis passed through a 500 liter resin column containing Rohm & Haas,Amberlite® IRA-743-A chelating resin. The resin uses moiety thatspecially attracts boron. Other companies make a similar suitable resinwhich may also be used in this application. The water is evaporated torecover the sugar which is then pyrolyzed at 1200 C in an inertatmosphere until the degree of graphitization is about 45%, resulting in400 kilograms of carbon. This material is briquetted into briquettes ofseveral sizes from about 2-6 inched on edge. The carbon briquettes andthe silica briquettes are feed into a submerged arc furnace. The furnaceis heated to over 1700 C and after 5 hours silicon metal starts tocollect at the bottom. The silicon metal is periodically tapped from thebottom of the furnace as needed. As the silicon is tapped additionalbriquettes of silica and carbon (made as above) may be added to operatethe furnace continuously for months at a time. The bottom tap andconduit for the molten silicon is covered by an inert atmosphere ofargon maintained in a box covering the bottom tap, conduit and molds forallowing the silicon to cool. After the silicon has solidified the moldmay be removed from the inert atmosphere. The silicon is then allowed tocrack or is crushed and bagged in a suitable container for shipping.

Alternatively, the molten silicon may be poured into a preheatedcrucible suitable for going directly into a directional solidificationfurnace and then solidified according to the cooling profile of thefurnace. This step will save considerable energy and time use on the DSfurnace since time and energy are not wasted on solidifying and thenre-melting the same silicon. This method will also be useful for formingmono-crystalline silicon through CZ pulling as well.

Example 8

1000 kilograms of sodium silicate is dissolved in 20000 liters ofde-ionized water. The pH of the solution is 11.2 The sample pH isadjusted to 10.5 with sulfuric acid. The sample is analyzed for boronusing both Inductively Coupled Plasma (ICP) as well as HACH wet borondetermination methods using BoroTrace™ 3 reagent. The original solutionis passed through a 500 liter resin column containing Rohm & Haas,Amberlite® IRA-743-A chelating resin. The resin uses moiety thatspecially attracts boron. Other companies make a similar suitable resinwhich may also be used in this application. A entire fraction iscollected and analyzed for boron. The solution from the column is nowtreated with sulfuric acid until the pH is 7.0. After standing a gelforms which is filtered and dried at up to 1400° C. The dried materialis briquetted into briquettes of several sizes from about 2-6 inched onedge. 400 kilograms of carbon black made from natural gas is dissolvedin 2000 liters of DI water the solution is passed through a 500 literresin column containing Rohm & Haas, Amberlite® IRA-743-A chelatingresin. The resin uses moiety that specially attracts boron. Othercompanies make a similar suitable resin which may also be used in thisapplication. The water is evaporated to recover the sugar which is thenpyrolyzed at 1200° C. in an inert atmosphere until the degree ofgraphitization is about 45%, resulting in 400 kilograms of carbon. Thismaterial is briquetted into briquettes of several sizes from about 2-6inched on edge. The carbon briquettes and the silica briquettes are feedinto a submerged arc furnace. The furnace is heated to over 1700° C. andafter 5 hours silicon metal starts to collect at the bottom. The siliconmetal is periodically tapped from the bottom of the furnace as needed.As the silicon is tapped additional briquettes of silica and carbon(made as above) may be added to operate the furnace continuously formonths at a time. The bottom tap and conduit for the molten silicon iscovered by an inert atmosphere of argon maintained in a box covering thebottom tap, conduit and molds for allowing the silicon to cool. Afterthe silicon has solidified the mold may be removed from the inertatmosphere. The silicon is then allowed to crack or is crushed andbagged in a suitable container for shipping.

Alternatively, the molten silicon may be poured into a preheatedcrucible suitable for going directly into a directional solidificationfurnace and then solidified according to the cooling profile of thefurnace. This step will save considerable energy and time use on the DSfurnace since time and energy are not wasted on solidifying and thenre-melting the same silicon. This method will also be useful for formingmono-crystalline silicon through CZ pulling as well.

Example 9

1000 kilograms of sodium silicate is dissolved in 20000 liters ofde-ionized water. The pH of the solution is 11.2 The sample pH isadjusted to 10.5 with sulfuric acid. The sample is analyzed for boronusing both Inductively Coupled Plasma (ICP) as well as HACH wet borondetermination methods using BoroTrace™ 3 reagent. The original solutionis passed through a 500 liter resin column containing Rohm & Haas,Amberlite® IRA-743-A chelating resin. The resin uses moiety thatspecially attracts boron. Other companies make a similar suitable resinwhich may also be used in this application. A entire fraction iscollected and analyzed for boron. The boron is reduced to less than 0.04ppm the detection limit of the ICP method. The solution from the columnis now treated with sulfuric acid until the pH is 7.0. After standing agel forms which is filtered and dried at up to 1400 C. The driedmaterial is briquetted into briquettes of several sizes from about 2-6inched on edge. Separately, 1000 kilograms of sugar is dissolved in 2000liters of DI water the solution is passed through a 500 liter resincolumn containing Rohm & Haas, Amberlite® IRA-743-A chelating resin. Theresin uses moiety that specially attracts boron. Other companies make asimilar suitable resin which may also be used in this application. Thesolution has 0.01 ppm of Boron. The water is evaporated to recover thesugar which is then pyrolyzed at 1200 C in an inert atmosphere until thedegree of graphitization is about 45%, resulting in 400 kilograms ofcarbon. This material is briquetted into briquettes of several sizesfrom about 2-6 inched on edge. The carbon briquettes and the silicabriquettes are feed into a submerged arc furnace. The furnace is heatedto over 1700 C and after 5 hours silicon metal starts to collect at thebottom. The silicon metal is periodically tapped from the bottom of thefurnace as needed. As the silicon is tapped additional briquettes ofsilica and carbon (made as above) may be added to operate the furnacecontinuously for months at a time. The bottom tap and conduit for themolten silicon is covered by an inert atmosphere of argon maintained ina box covering the bottom tap, conduit and molds for allowing thesilicon to cool. After the silicon has solidified the mold may beremoved from the inert atmosphere. While the silicon is still moltenapproximately 10 ppm, (15 grams) Misch metal is added and stirred intothe silicon. When the silicon is directionally solidified the rare earthmetal will be transferred to the cold end (usually the top) of the DSproduct along with impurities such as boron, oxygen. Nitrogen and oxygenresulting in an even higher purity silicon. The silicon is then allowedto crack or is crushed and bagged in a suitable container for shipping.Alternatively, the molten silicon may be poured into a preheatedcrucible suitable for going directly into a directional solidificationfurnace and then solidified according to the cooling profile of thefurnace. This step will save considerable energy and time use on the DSfurnace since time and energy are not wasted on solidifying and thenre-melting the same silicon. This method will also be useful for formingmono-crystalline silicon through CZ pulling as well.

1. A method of producing silicon having high purity suitable forphotovoltaic cells, the method comprising reducing silica pre-purifiedin an aqueous solution with carbonaceous agent where the pre-purifiedsilica has a low amount of boron suitable for photovoltaic cells.
 2. Amethod in claim 1 where the silica is obtained via a purification methodusing an aqueous solution of a water soluble form of silica wherein theaqueous solution being filtered to remove solid impurities and treatedwith a boron specific chelating resin.
 3. The method according to claim2, wherein the boron specific chelating resin is an ion exchange resinhaving a functional group of N-methylglucamine.
 4. The method accordingto claim 2 wherein a transition metal, calcium or magnesium is added tothe aqueous solution of the water soluble form of silica before thefiltration.
 5. The method according to claim 2 wherein the aqueoussolution of the water soluble form of silica is treated with molybdatesalt or molybdate salt treated anion resin.
 6. The method according toclaim 1, wherein the water soluble form of silica is an alkali silicate.7. The method according to claim 1, wherein the carbonaceous agent issubstantially free of boron and phosphorus.
 8. The method according toclaim 7, wherein the carbonaceous agent is prepared from a flammablegas.
 9. The method according to claim 8, wherein the flammable gas isnatural gas, methane, ethane, acetylene, ethylene, propane, propene,allene, butane, LPG or in general any C1-C4 gases that are substantiallyfree of boron and phosphorus.
 10. The method according to claim 7,wherein the carbon is prepared from a carbohydrate, starch or othercarbonaceous material.
 11. The method according to claim 9, wherein thecarbon is prepared by a method comprising dissolving the carbohydrate inwater; purifying the resulting solution; removing water; and pyrolyzingthe purified carbohydrate to obtain the carbon.
 12. The method accordingto claim 11, wherein the pyrolization is conducted to obtain the degreeof graphitization of about 40-45%.
 13. The method according to claim 12,wherein the desired degree of graphitization is obtained by mixingvarious carbon forms.
 14. The method according to claim 7, wherein thecarbon is treated with chlorine to remove B and P.
 15. The methodaccording to claim 7, wherein the pre-purified silica is in acrystalline form.
 16. The method according to claim 7, wherein thepre-purified silica and or the carbon is in a briquetted form.
 17. Themethod according to claim 7, wherein the pre-purified silica and thecarbon are prepared by co-precipitation so that the silica and thecarbon are uniformly mixed.
 18. The method according to claim 1, whereinthe pre-purified silica has a boron content about 5 ppm or less.
 19. Themethod according to claim 1, wherein the pre-purified silica has a boroncontent about 1 ppm or less.
 20. The method according to claim 1, wherethe purification method is conducted in a basic condition.
 21. Themethod according to claim 7, wherein the reduction reaction is carriedout using a liner in a submerged arc furnace wherein molten high puritysilicon is formed.
 22. The method according to claim 21, wherein thefurnace is made from refractory materials with an extremely low boronand phosphorus content.
 23. The method according to claim 21, whereinthe furnace uses high purity graphite rods for the electrodes.
 24. Themethod according to claim 21, wherein the furnace has a bottom tap toremove the molten high purity silicon from the bottom.
 25. The methodaccording to claim 21, wherein exterior to the furnace at the point ofmolten silicon discharge has an inert gas chamber for allowing themolten silicon to cool in an inert gas so that the silicon does notreact with oxygen in the atmosphere.
 26. The method according to claim21, wherein the molten silicon is further treated with a rare-earthmetal to remove carbon, oxygen, nitrogen, boron and other impuritiesfrom the silicon.
 27. A method for preparing high purity siliconcomprising (a) obtaining an aqueous silicate solution; (b) filtering thesolution; (c) passing the filtrate through an boron specific ion resincolumn; (d) converting the silicate to silica; (e) reducing the silicawith carbon having low content of boron and phosphorous in a furnace;and (f) cooling the resulted molten high purity silicon.
 28. The methodaccording to claim 27, further comprising converting low purity silica,sand, quartz or other silica containing materials or silicon or siliconmonoxide to alkali silicate.
 29. The method according to claim 27,wherein the carbon is prepared by pyrolizing a carbohydrate
 30. A methodpreparing high purity silica having low B and P contents, the methodcomprising: (a) obtaining aqueous silicate solution; (b) optionallyadding a transit metal, calcium, or magnesium; (c) filtering thesolution; (d) passing the filtered through an boron specific ion resincolumn; (e) removing the water; and (f) converting the resulted silicatesalt to silica.
 31. The method according to claim 30, further comprisingtreating the aqueous silicate solution with an anion exchange resin thatis pre-treated with ammonium molybdate or tungstate.
 32. The methodaccording to claim 1 wherein the product is high purity silicon carbide33. The method according to claim 1 wherein the product is high puritysilicon monoxide.
 34. The method according to claim 1, wherein thecarbonaceous agent has a boron content about 1 ppm or less.
 35. Themethod according to claim 1 where the reducing agent is an activepurified metal such as aluminum or magnesium instead of carbon.
 36. Themethod for purifying low grade silicon to high purity silicon suitablefor photovoltaic cells, the method comprising: (a) converting the lowgrade silicon to a water soluble form; (b) purifying the water solubleform of silica in an aqueous solution; (c) converting the purified watersoluble form of silica to silica; and (d) converting the resultingsilica to the high purity silicon.